1. Field of the Invention
The present invention relates to reflective and transflective liquid crystal display (LCD) devices and more particularly, to reflective and transflective LCD devices using a dispensing method and a fabricating method thereof.
2. Discussion of the Related Art
Generally, liquid crystal display (LCD) devices include upper and lower substrates, where color filters and thin film transistors (TFTs) are respectively disposed. A liquid crystal layer is interposed between the upper substrate and the lower substrate. Transmittance of the LCD devices is controlled by applying a voltage to common electrodes and pixel electrodes so that characters and images are displayed with a light shutter effect.
A fabrication process of a liquid crystal cell will be explained briefly.
After the upper and lower substrates are aligned and attached so that the surfaces of the common electrodes and pixel electrodes face each other, the liquid crystal material is injected between the substrates through an injection hole and the injection hole is then sealed. A polarizer is then attached to each outer surface of the upper and lower substrates.
The fabrication process of the liquid crystal cell seldom includes repeated steps compared with the fabrication processes of the TFT and the color filter. The process includes forming an orientation film, forming a cell gap and cutting the cell.
FIG. 1 is a flow chart illustrating a conventional fabrication process of a liquid crystal cell.
In step st1, a lower substrate is prepared by forming an array of TFTs and pixel electrodes on the lower substrate.
In step st2, an orientation film is formed on the lower substrate. Formation of the orientation film includes depositing a polymeric thin film on the substrate and subsequently performing a uniform rubbing process. The rubbing process determines an initial alignment direction and supplies the normal operation of the liquid crystal layer and the uniform display characteristic of the LCD device. Typically, an organic material of the polyamide series is used as the orientation film. The rubbing method includes rubbing the orientation film in a specific direction with a rubbing cloth, thereby aligning the liquid crystal molecules along or in the rubbing direction.
In step st3, a seal pattern is formed on the lower substrate. In the liquid crystal cell, the seal pattern serves two functions: forming a gap for the injection of the liquid crystal material and confining the injected liquid crystal material. The seal patterning process forms a desired pattern by the application of a thermosetting plastic. A screen-printing method using a screen mask is typically used for this process.
In step st4, a spacer is sprayed on the lower substrate. The size of the spacer used in the liquid crystal cell maintains a precise and uniform gap between the upper and lower substrates. Accordingly, the spacers are uniformly sprayed on the lower substrate. The spacer spray method can be divided into two different types: a wet spray method that involves spraying a mixture of alcohol and spacer material and a dry spray method that involves spraying spacer material alone. Furthermore, the dry spray method can be sub-divided into two different types: an electrostatic spray method that uses electrostatic force, and a non-electric spray method that uses gas pressure. Since the liquid crystal cell structure is susceptible to damage from static electricity, the non-electric method is widely used.
In step st5, the upper and lower substrates are aligned and attached. The alignment margin between the upper and lower substrates is determined by the device design, and accuracy within a few micrometers is generally required. If the alignment margin is exceeded, the liquid crystal cell will not operate adequately due to light leakage.
In step st6, the attached liquid crystal substrate is divided into unit cells. Generally, a plurality of unit cells are formed on a large sized glass substrate and then divided through a cutting process. In the fabrication process of the initial LCD devices, the unit cells are separated after simultaneous injection of the liquid crystal material into the unit cells. However, injection of liquid crystal material is commonly performed after a large sized liquid crystal substrate is cut into unit cells due to an increase in the cell size. The cell cutting process includes a scribe process that forms cutting lines on a surface of the substrate using a diamond pen, the hardness of which is higher than that of the glass substrate, and a breaking process that divides the substrate by force.
In step st7, a liquid crystal material is injected into the unit cells. The unit cell has a size of several hundred square centimeters with a gap of several micrometers. Accordingly, a vacuum injection method using pressure difference between the interior and exterior of the unit cell is commonly used as an effective injection method.
The injection method of liquid crystal material is classified into a dip method or a contact method. In the dip method, the injection hole is dipped into a liquid crystal tank under a vacuum state and the liquid crystal material is injected due to a pressure difference between interior and exterior of an LCD panel. In the contact method, the injection hole contacts the surface of the liquid crystal material in the liquid crystal tank. However, the injection method using a pressure difference under a vacuum takes a long period of time and also the injection hole may become contaminated.
To solve the above problems, a dispensing method is suggested. In the dispensing method, a sealant is printed at a boundary of a plurality of cells on an array substrate and then the liquid crystal material is sufficiently dropped in a region defined by the sealant using a means such as a dispenser. A process time is reduced and the production yield is dramatically improved because the liquid crystal layer is formed in the LCD panel in a short period of time.
FIGS. 2A to 2F are schematic plan views and cross-sectional views showing the fabricating process of a liquid crystal cell.
In FIGS. 2A and 2B, a first substrate 2 having a plurality of liquid crystal cells “A” is provided. The first substrate 2 has a sub-color filter corresponding to a pixel electrode of a second substrate and a black matrix corresponding to a region between the adjacent pixel electrodes.
In FIGS. 2C and 2D, a sealant 6 is printed on a second substrate 4 at regions corresponding to the boundary of each liquid crystal cell “A.” The second substrate 4 has an array line, a pixel electrode and a switching device, i.e., a thin film transistor (TFT). Next, a liquid crystal layer is formed at an interior 8 of the sealant with a dispenser.
In FIGS. 2E and 2F, the first substrate 2 is attached to the second substrate 4 having the liquid crystal layer to form a liquid crystal panel 10. Then a shielding mask 12 is disposed over the liquid crystal panel 10. The shielding mask 12 includes a transmission region “B” and a shield region “C.” The transmission region “B” corresponds to the sealant 6 of the liquid crystal panel 10 and the shield region “C” corresponds to the interior of the sealant 6, i.e., a display area “D” of the liquid crystal panel 10. The display area “D” is shielded to prevent a channel of the TFT from being exposed to ultraviolet (UV) light during the hardening process of the sealant. Even though the first substrate has the black matrix corresponding to the TFT, the incident light may be scattered by a reflective plate and influence the channel. Accordingly, the shielding mask should be used for preventing this influence.
After hardening the sealant 6 with UV light, the first and second substrates 2 and 4 are completely attached through a hot press process. The attached substrates are then cut into a plurality of unit cells.
However, when the dispensing method is used for the forming process of the liquid crystal layer, the UV light should be irradiated to the sealant for hardening. Therefore, the black matrix should not be formed on the first substrate at a region corresponding to the sealant. Therefore, to prevent the pixel region from being exposed to UV light, larger margins are necessary when a boundary of the liquid crystal panel is designed.
FIG. 3 is a schematic plan view of a conventional transflective LCD device where a liquid crystal layer is formed through the dispensing method and FIG. 4 is a cross-sectional view taken along a line IV—IV of FIG. 3.
In FIGS. 3 and 4, a second substrate 14, referred to as an array substrate, for a transflective LCD device has a pixel region “P” including a transmissive portion “E” and a reflective portion “F.” The transmissive portion “E” and the reflective portion “F” have a transflective electrode formed by overlapping a transparent electrode 16 and a reflective electrode 20 having a transmissive hole 18. The pixel region “P” is defined by a gate line 22 and a data line 24 crossing the gate line 22. A gate pad 23 and a data pad (not shown) having a specific area are disposed at an end of the gate line 22 and the data line 24, respectively. A TFT “T” including a gate electrode 26, an active layer 32, and source and drain electrodes 28 and 30 are connected to the gate and data lines 22 and 24.
A black matrix 36 and a color filter 37 are formed on a first substrate 34, referred to as a color filter substrate, said first substrate corresponding to the second substrate 14. The black matrix 36 corresponding to the TFT “T” of the second substrate 14 shields an active layer 32 from incident light. Since a sealant 6 is an UV curable resin, the sealant should be irradiated by UV light and thus the black matrix 36 should not be formed over the sealant 6.
However, in the transflective LCD panel of the above structure, since the black matrix is formed on the first substrate, the aperture ratio decreases. Moreover, since the black matrix is not formed at a region corresponding to the sealant, larger margins are necessary to protect the display region when a boundary of the liquid crystal panel is designed. Therefore, it is difficult to apply the transflective LCD panel of the above structure to a compact product such as a mobile phone. Further, the fabrication cost are increased due to the use of an additional shielding mask. These problems do not occur only in a transflective LCD device but also in a reflective LCD device.